A voltage controlled oscillator (VCO) is a common circuit used to generate, among other things, clock signals for use in data processors, radio transceivers, and similar circuits. Typically, is the VCO is part of a phase-locked loop (PLL) that uses feedback to provide an accurate and stable clock reference signal for other circuits. The frequency of oscillations generated by a VCO are controlled by an externally applied control voltage. Two important parameters in VCO design are linearity and sweep range. Linearity correlates the change in frequency or the VCO output to the change in the control voltage. The sweep range is the range of possible frequencies produced by VCO control voltage. A problem for many voltage controlled oscillators is that there is a trade-off between the sweep range and linearity at low frequency. For any appreciably large sweep range, a conventional VCO typically exhibits non-linearity at low frequency.
Therefore, there is a need in the art for an improved voltage controlled oscillator that has improved linearity at low frequency. More particularly, there is a need in the art for a voltage controlled oscillator that has a large sweep range while maintaining high linearity at low frequency.